Mobile devices, such as but not limited to personal data appliances, cellular phones, radios, pagers, lap top computers, and the like are required to operate for relatively long periods before being recharged. These mobile devices usually include one or more processors as well as multiple memory modules and other peripheral devices.
In order to reduce the power consumption of mobile devices various power consumption control techniques were suggested. A first technique includes reducing the clock frequency of the mobile device. A second technique is known as dynamic voltage scaling (DVS) or alternatively is known as dynamic voltage and frequency scaling (DVFS) and includes altering the voltage that is supplied to a processor as well as altering the frequency of a clock signal that is provided to the processor in response to the computational load demands (also referred to as throughput) of the processor. Higher voltage levels are associated with higher operating frequencies and higher computational load but are also associated with higher energy consumption.
The power consumption of a transistor-based device is highly influenced by leakage currents that flow through the transistor. The leakage current is responsive to various parameters including the threshold voltage (Vt) of the transistor, the temperature of the transistor, and the like. Transistors that have higher Vt are relatively slower but have lower leakage currents while transistors that have lower Vt are relatively faster but have higher leakage current.
The various transistors may also differ by the amount of current they are able to drive. Typically, input/output (I/O) circuitry includes transistors that are capable of driving relatively large current. Due to temperature changes as well as process variations the impedance of I/O transistors may vary and may result in impedance mismatch that also results in power waste.
FIG. 1 illustrates the behavior of two transistor types as well as a memory device. First and second voltage regions 12 and 14 describe the behavior of a typical low threshold voltage transistor. First voltage region 12 starts at about zero volts and ends at a third voltage level VL3. The second voltage region starts at VL3 and ends at a high voltage level, such as a maximal voltage level VDD. The low threshold voltage transistors operate at the second voltage region 14, and not expected to operate at the first voltage region 12.
The third and fourth voltage regions 22 and 24 describe the behavior of a typical high threshold voltage transistor. Third voltage region 22 starts at about zero volts and ends at a first voltage level VL1. The fourth voltage region 24 starts at VL1 and ends at a high voltage level, such as a maximal voltage level VDD.
The high threshold voltage transistors operate at the fourth voltage region 24 and are not expected to operate at the third voltage region 22.
The fifth, sixth and seventh voltage regions 32, 34 and 36 describe the behavior of a typical memory device. The fifth voltage region 32 starts at about zero volts and ends at a second voltage level VL2. The memory device must receive a voltage that exceeds VL2 in order to store information. The sixth voltage region 34 starts at VL2 and ends at a fourth voltage level VL4. The memory device must receive a voltage that exceeds VL4 in order to allow read and write operations. The seventh voltage region 36 starts at VL4 and ends at a high voltage level, such as a maximal voltage level VDD.
The second, fourth and seventh voltage regions 14, 24 and 36 can be regarded as optimal regions as they provide a compromise between power consumption and performance. In many system transistors of various types, such as the three mentioned above types are included within a single voltage/frequency region, thus they receive the same voltage as well as the same clock signal.
It can be described, that voltage VL1 is about 0.5 volts, VL2 is about 0.6 volts, VL3 is about 0.7 volts, VL4 is about 0.9 volts and VDD is about 1.5 volts. It is noted that these values are provided as an example only. It is further noted that even when using the same integrated circuits as the inventors used said values might vary in response to various conditions such as temperature changes and process variations.
There is a need to provide a method and an apparatus for providing voltage and clock signals to a system that includes transistors of various types.